The design and optimization of Organic Rankine Cycle (ORC) require knowledge concerning the thermophysical properties of the working fluids: pure components or mixtures. These properties are generally calculated by thermodynamic and transport property models (thermodynamic or equation of state or correlations). The parameters of these models are adjusted on accurate experimental data. The main experimental data of interest concern phase equilibrium properties (noncritical and critical data), volumetric properties (density and speed of sound), energetic properties (enthalpy, heat capacity), and transport properties (dynamic viscosity and thermal conductivity). In this chapter, some experimental techniques frequently used to obtain the experimental data are presented. Also, we will present some models frequently used to correlate the data and some results (comparison between experimental data and model predictions).